Method for forming pattern

ABSTRACT

In an exposure step, a combination of a first photomask and a second mask is used. The first mask has a real pattern corresponding to the pattern actually formed on the film to be processed, and a dummy pattern added for controlling pattern pitch in the first photomask within a prescribed range; and the second photomask has a pattern isolating a real-pattern-formed region from a dummy-pattern-formed region. In forming the pattern, after forming a film to be processed on a substrate, a first mask is formed on the film to be processed, by lithography, using the first photomask, and a second mask is formed on the film to be processed, by lithography, using the second photomask. Thereafter, the film to be processed is etched and removed using the first and second masks as masks to form the pattern.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to photomasks, and a method for forming apattern. More specifically, the present invention relates to photomasksused for forming fine patterns on a film to be processed usinglithography, and a method for forming patterns using such photomasks.

2. Background Art

In recent years, with the high integration and miniaturization ofsemiconductor devices, the improvement of resolution in thephotolithography has been demanded. In the photolithography, a criticalresolution R, which is the critical pattern dimension that can beresolved, is represented by the following equation (1):R=k ₁·λ/(NA)  (1)where k₁ is a constant dependent on image-forming conditions and resistconditions, λ (nm) is the wavelength of exposing light, and NA is thenumerical aperture of the taking lens.

Therefore, in order to improve resolution, the wavelength λ of exposinglight should be shortened, or the numerical aperture of the lens shouldbe enlarged. However, the pattern size required by exposing techniqueshas been diminished, and the formation of patterns smaller than therealizable critical resolution R determined by the wavelength λ ofexposing light and the numerical aperture of the lens NA has beendemanded.

For example, in a semiconductor device having a multi-layerinterconnection structure, the formation of a 1:1 line-and-space pattern(hereafter referred to as L/S pattern), which is the same as the gatepitch, is required in the area in the vicinity of the lowermost layer,that is, in the vicinity of the contact hole. For example, since thegate pitch is about 130 nm in the 65-nm technological node, theformation of the L/S pattern having this pitch is required in thevicinity of the lowermost layer of such a semiconductor device. However,it is difficult to cope with such a fine pattern only by shortening thewavelength and the enlarging numerical aperture.

Therefore, in order to form a pattern finer than the critical resolutionR of the exposure apparatus, the combination of the technique known asresolution enhancement technique with shortening the wavelength and theenlarging numerical aperture has been considered. There are twotechniques of resolution enhancement, for illumination and for the mask.

Specifically, the resolution enhancement for illumination is a techniqueusing an off-axis illumination method. This method improves resolutionby adding an aperture under the exposing light source, and for example,the annular illumination, which is one of the techniques of the off-axisillumination methods, by shielding the center portion of the luminousflux to decrease the image components by three-beam interference, andincrease two-flux interference components.

On the other hand, a resolution enhancement technique for the mask usesa phase-shifting mask. While a conventional chromium mask controls onlythe amplitude of light, the phase-shifting mask improves resolutionutilizing the phase contrast of light. The examples of phase-shiftingmasks include an attenuated phase-shifting mask and an alternating(Levenson-type) phase-shifting mask (e.g., refer to Japanese PatentApplication Laid-Open No. 7-181668).

The combination of these two resolution enhancement techniques, i.e.,off-axis illumination and the phase-shifting mask, is often used for theformation of fine patterns.

The off-axis illumination method depends on the layout of the pattern,considerably. For example, for only lines running in the verticaldirection or in the horizontal direction, a dipole illuminating lightsource is effective for the strong contrast of light. When there areline patterns in both vertical and horizontal directions, a quadrapoleilluminating light source is effective. Furthermore, when there are nolimitations in the direction or the angle of the lines, annularillumination is effective.

The optimum location of the aperture depends on the pattern pitch of thecycle pattern. Therefore, the off-axis illumination method isparticularly effective for cyclic patterns, but the effect is reducedunless the pattern pitch is made constant to some extent. For patternsthat are not periodic, for example, a pattern having different pitches,an isolated pattern, or a pattern located in the end of cyclic pattern,the light intensity profiles become significantly different. Therefore,for patterns having no periodicity, resolution or focal depth is madeworse rather than being improved as compared to the use of ordinaryillumination.

As a measure for correcting dimensional differences in patterns that arenot periodic, for example, the use of OPC (optical proximity correction)can be considered. However, although OPC is effective for the correctionof dimensions, process margin, such as focal depth and exposure margin,is left unchanged.

FIG. 44 is a schematic diagram for illustrating a photomask whereon apattern is formed. FIG. 45 is a graph for illustrating the relationshipbetween defocus (μm) and the dimension (nm) of each transferred linepattern, when the pattern of the photomask is transferred.

Here, the photomask is an attenuated phase shifting mask having atransmittance of 5% whereon an L/S pattern of 130-nm pitch is laid out.

Here, since dimensional correction is performed by OPC, the dimensionsof lines at best focus are substantially uniform, each other, as FIG. 45shows. However, in the case of defocus, although the dimensions of Line2 and Line 3 in the center portion in periodicity pattern are notchanged, the dimensions of Line 1 in the end portion and isolated Line 4are significantly changed to be smaller.

When the conventional off-axis illumination method or phase shiftingmasks are used, the process margin for the pattern out of periodicity islow, and even if dimensional correction by OPC is used, there islimitation in the accurate transfer of patterns. Therefore, in thepattern portion whereon a pattern out of periodicity is transferred,difference from the designed pattern dimension increases, and therefore,defects such as short-circuiting and disconnection often occur in thisarea, causing problems.

SUMMARY OF THE INVENTION

The present invention solves the above problems, and provides aphotomask, and a method for forming patterns in order to transferpatterns more accurately to a film to be processed even when patternsout of periodicity are formed.

According to one aspect of the present invention, a pair of photomasksused in photolithography comprises a first photomask, and a secondphotomask. The first photomask comprises a real pattern and a dummypattern. The real pattern is an actual pattern formed on a film to beprocessed. The dummy pattern added so as to control the pattern pitch ofthe first photomask to be within a prescribed range. The secondphotomask comprises a pattern isolating a region wherein the dummypattern is formed from a region wherein the real pattern is formed.

According to another aspect of the present invention, in a method forforming patterns, a film to be processed is formed on a substrate, and afirst and a second masks are formed on the film to be processed bylithography using a first and a second photomasks, respectively. Thefilm to be processed is etched using the first mask and the second maskas masks. Here, the first photomask has a real pattern, which is anactual pattern formed on the film to be processed, and a dummy patternadded so as to control the pattern pitch in the first photomask to bewithin a prescribed range. The second photomask has a pattern isolatinga region wherein the dummy pattern is formed from the region wherein thereal pattern is formed.

Other and further objects, features and advantages of the invention willappear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view for illustrating a first photomask in the firstembodiment of the present invention;

FIG. 2 is a top view for illustrating a second photomask in the firstembodiment of the present invention;

FIGS. 3A and 3B are schematic diagrams for illustrating the trenchpattern formed using the first and second photomasks in the firstembodiment of the present invention; and FIG. 3A illustrates the topsurface, and FIG. 3B illustrates the cross section along the broken lineIIIB-IIIB in FIG. 3A;

FIG. 4 is a flow diagram for describing the method for forming thetrench patterns 24 in the first embodiment of the present invention;

FIGS. 5 to 13 are schematic sectional views for illustrating the stateof the trench patterns 24 in each forming step in the first embodimentof the present invention;

FIG. 14 is a top view for illustrating a first photomask in the secondembodiment of the present invention;

FIG. 15 is a top view for illustrating a second photomask in the secondembodiment of the present invention;

FIG. 16 is a schematic diagram for illustrating the state wherein thefirst photomask is superimposed on the second photomask in the secondembodiment of the present invention;

FIG. 17 is a graph for illustrating the relationship between defocus andthe line size of the line patterns formed in the second embodiment ofthe present invention;

FIG. 18 is a top view for illustrating a first photomask in the thirdembodiment of the present invention;

FIG. 19 is a top view for illustrating a second photomask in the thirdembodiment of the present invention;

FIG. 20 is a schematic diagram for illustrating the state wherein thesecond photomask is superimposed on the first photomask according tothird embodiment of the present invention.

FIG. 21 is a flow diagram for illustrating the method for formingpatterns according to the fourth embodiment of the present invention.

FIGS. 22 to 27 are schematic sectional views for illustrating the statesin the process for forming patterns according to the fourth embodimentof the present invention;

FIG. 28 is a schematic sectional view for illustrating the method formanufacturing a semiconductor device according to the fifth embodimentof the present invention;

FIG. 29 is a flow diagram for illustrating the method for manufacturingthe semiconductor device according to the fifth embodiment of thepresent invention;

FIGS. 30 to 43 are schematic sectional views for illustrating the statein each step for manufacturing the semiconductor device according to thefifth embodiment of the present invention;

FIG. 44 is a schematic diagram for illustrating a conventional photomaskwhereon a pattern is formed; and

FIG. 45 is a graph for illustrating the relationship between defocus(μm) and the dimension (nm) of each transferred line pattern, when thepattern of the photomask as illustrated in FIG. 44 is transferred.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments of the present invention will be described belowreferring to the drawings. In the drawings, the same or correspondingparts will be denoted by the same reference numerals, and thedescription thereof will be simplified or omitted.

First Embodiment

FIG. 1 is a top view for illustrating a first photomask in the firstembodiment of the present invention; and FIG. 2 is a top view forillustrating a second photomask in the first embodiment of the presentinvention.

As FIG. 1 illustrates, the first photomask is an attenuated phaseshifting mask having a transmittance of 5%.

In the first photomask, a light-shielding portion 2 having atransmittance of 5%, real patterns 4 and dummy patterns 6, which areopenings formed in the light-shielding portion 2, are formed. The realpatterns 4 are patterns corresponding to trench patterns formed on thefilm subjected to processing; and the dummy patterns 6 are patternsadded to the real patterns 4 so that the pitch of line patterns becomesconstant to some extent in the entire first photomask 100. In the firstphotomask combining the real patterns 4 and the dummy patterns 6, a 1:1line-and-space pattern (hereafter referred to as L/S pattern) of apattern pitch of 130 nm is formed. Here, the pattern pitch means thedistance from a line pattern to the adjacent line pattern, and here, itis the value of the width of a line pattern added to the width of thespace between line patterns.

In the second photomask, a chromium light-shielding portion 8 and anopening portion 10 are formed. The opening portion 10 is formed so as tosurround the region where the real patterns 4 of the first photomask areformed. The light-shielding portion 8 is formed so as to correspond tothe peripheral light-shielding portion 2 where the pattern of the firstphotomask is not formed, and the area where the dummy patterns 6 areformed.

Specifically, when the first photomask is superimposed on the secondphotomask, only the area of the real patterns 4 is opened.

FIGS. 3( a) and 3(b) are schematic diagrams for illustrating the trenchpattern formed using the first and second photomasks in the firstembodiment of the present invention; and FIG. 3( a) illustrates the topsurface, and FIG. 3( b) illustrates the cross section along the brokenline A-A′ in FIG. 3( a).

As FIG. 3A and 3B illustrate, a low- dielectric-constant insulating film22 is formed on the substrate 20. The low- dielectric-constantinsulating film 22 is a film to be processed in the first embodiment. Inthe low- dielectric-constant insulating film 22, trench patterns 24 areformed using the first and second photomasks.

As FIG. 3A illustrates, the trench patterns 24 are patterns thatcorrespond to the real patterns 4 of the first photomask. In otherwords, the trench patterns 24 are patterns that correspond to thepatterns formed when the first photomask is superimposed on the secondphotomask.

FIG. 4 is a flow diagram for describing the method for forming thetrench patterns 24 in the first embodiment of the present invention.FIGS. 5 to 13 are schematic sectional views for illustrating the stateof the trench patterns 24 in each forming step.

The method for forming the trench patterns 24 in the first embodiment ofthe present invention will be specifically described below referring toFIGS. 4 to 13.

First, as FIG. 5 illustrates, a low-dielectric-constant insulating film22 is vapor-deposited on the substrate 20 using plasma CVD (chemicalvapor deposition) (Step S2). Next, a silicon nitride film 30 isvapor-deposited on the low-dielectric-constant insulating film 22 (StepS4). Here, the silicon nitride film 30 is formed to have a thickness ofabout 80 nm using plasma CVD method. The silicon nitride film 30 is amaterial film later patterned to be a first hard mask. Thereafter, anorganic anti-reflection coating 32 is formed (Step S6), and a positiveresist 34, which is a positive photosensitive material, is formedthereon (Step S8). As the resist 34, for example, a fluorine main-chainpositive resist for F₂ lithography can be applied using spin coating.

Next, as FIG. 6 illustrates, the positive resist 34 is exposed (StepS10). Here, a dipole illustration light source of a center sigma σ of0.4 and the radius of σ of 0.05 using an F₂ excimer laser of awavelength of 157.6 nm as the exposing light source is used. The numeralaperture NA of the lens is 0.95. As the photomask, the above-describedfirst photomask is used.

Thereafter, a developing process is carried out (Step S12), and heattreatment is performed as required. Thereby, the patterns correspondingto the first photomask are transferred to the positive resist 34. Inother words, openings (real) 36 corresponding to the real patterns 4,and openings (dummy) 38 corresponding to the dummy patterns 6 are formedin the positive resist 34.

Next, as FIG. 7 illustrates, dry etching is performed using the patternsof the positive resist 34 as masks (Step S14). As the etching gas, forexample, a mixed gas of carbon tetrafluoride, oxygen and argon is used.Thereby, the organic anti-reflection coating 32 and the silicon nitridefilm 30 are etched, and the openings 36 and 38 penetrate the organicanti-reflection coating 32 and the silicon nitride film 30 to expose thesurface of the low-dielectric-constant insulating film 22 on the bottomsof the openings 36 and 38.

Next, the positive resist 34 and the organic anti-reflection coating 32are removed (Step S16). Thereby, a first hard mask 40 composed of thesilicon nitride film 30 is formed on.

Next, as FIG. 8 illustrates, a silicon oxide film 42 is vapor-depositedon the first hard mask 40 and the low-dielectric-constant insulatingfilm 22 (Step S18). The silicon oxide film 42 is formed using a plasmaCVD method so as to have a thickness of about 30 nm. The silicon oxidefilm 42 is a material film later patterned to be a second hard mask.

Next, as FIG. 9 illustrates, an organic anti-reflection coating 44 isformed on the silicon oxide film 42 (Step S20), and a positive resist 46is applied thereto (Step S22). The positive resist 46 is a fluorinemain-chain positive resist for F₂ lithography similar to theabove-described positive resist 34, and can be applied using spincoating.

Next, the positive resist 46 is exposed (Step S24). Here, the exposureis performed using an F₂ excimer laser as the exposing light source, andusing the above-described second photomask. Thereafter, a developingprocess is carried out (Step S26), and heat treatment is performed asrequired. Thereby, as FIG. 10 shows, an opening 48, which opens on theregion where openings (real) 36 are formed, is formed in the positiveresist 46. The opening 48 corresponds to the openings 10 of the secondphotomask.

Next, as FIG. 11 shows, the organic anti-reflection coating 44 and thesilicon oxide film 42 are subjected to dry etching using the positiveresist 46 as a mask (Steps S28 and S30). Here, as the etching gas, forexample, a mixed gas of cyclobutane octafluoride (C₄F₈), oxygen andargon is used. This etching gas provides a sufficient large etchingselectivity between the silicon nitride film 30 and the silicon oxidefilm 42. Therefore, when the silicon oxide film 42 is etched using thisgas, the first hard mask 40 can be left without being etched.

Next, the positive resist 46 and the organic anti-reflection coating 44are removed (Step S32). Thereby, as FIG. 12 illustrates, a first hardmask 40 and a second hard mask 50 are formed on thelow-dielectric-constant insulating film 22. Here, the openings to exposethe surface of the low-dielectric-constant insulating film 22 are onlyopenings (real) 36 that are portions corresponding to the real patterns4 of the first photomask, and the openings (dummy) 38 of the first hardmask 40 are covered with the second hard mask 50.

Next, as FIG. 13 illustrates, the low- dielectric- constant insulatingfilm 22 is subjected to dry etching using the first hard mask 40 and thesecond hard mask 50 (Step S34). Thereafter, the first hard mask 40 andthe second hard mask 50 are removed (Step S36). Thereby, as FIGS. 3A and3B illustrate, trench patterns 24 are formed on thelow-dielectric-constant insulating film 22.

In the first embodiment, as described above, the first photomask whereinthe pattern pitch is uniformed by adding dummy patterns 6 to realpatterns 4 was used as an attenuated phase shifting photomask, and adipole illuminating light source was used for exposure. Thereby,periodical fine patterns can be accurately transferred on the resist 34,and a first hard mask 40 having pattern dimensions faithful to thepattern design can be formed.

Thereafter, a second hard mask 50 can be formed on the first hard mask40 using the second photomask having an opening 10 only in the regionwhere required real patterns 4 are formed. Here, the second photomaskhas a relatively simple pattern that partitions the region where realpatterns 4 are formed from the region where dummy patterns 6 are formed,and therefore, the patterns can be relatively accurately transferredeven using ordinary exposure.

In addition, using a two-layer hard mask formed by superimposing thesecond hard mask 50 on the first hard mask 40 as the mask, thelow-dielectric-constant insulating film 22 can be etched.

Therefore, even when the pattern pitch is not periodical, or when thereis an isolated pattern, resolution enhancement techniques, such asoff-axis illumination and a phase shifting mask can be utilized in theportions where fine processing is required; therefore, fine patterns canbe accurately formed.

In the first embodiment, although only two trench patterns 24 are shownin the drawing for simplification of description, a plurality ofpatterns can be formed in a plurality of locations as required.

The first and second photomasks are not limited to those shown in FIGS.1 and 2. In the present invention, it is sufficient as long as the firstphotomask is formed by adding dummy patterns to real patterns so as touniform the pattern pitch to some extent; and it is sufficient as longas the second photomask covers the dummy pattern portion of the firstmask.

As the first photomask, an attenuated phase shifting mask was used.However, the present invention is not limited thereto, but for example,an alternating phase shifting mask may also be used. Depending on thesize of patterns to be formed, a chromium mask may also be used. As thesecond photomask, a chromium mask was used. However, if the periodicityof the pattern on the second photomask is considered, a phase shiftingmask as in the first photomask may also be used.

In the first embodiment, the case wherein a dipole illumination lightsource using an F₂ excimer laser was used for exposure in the formationof the first photomask 40 was described. However, the present inventionis not limited thereto, and a light source have another wavelength mayalso be used. Further, the light source is not limited to a dipoleillumination light source and another off-axis illumination lightsource, such as a quadrapole illumination light source and an annularillumination light source, may also be used, or a light source withoutdeformation may also be used. Here, any of a dipole illumination lightsource, a quadrapole illumination light source, and a annularillumination light source is suitable for the formation of line patternsusing an attenuated phase shifting mask as the first photomask, and theconditions of such light sources can be suitably selected consideringthe type, the pattern shape, the pattern size and the like of the firstphotomask.

In the first embodiment, a low-dielectric-constant insulating film 22was used as a film to be processed. However, the present invention isnot limited thereto, but can be applied to the patterning of otherfilms.

In the first embodiment, the case wherein a silicon nitride film 30 wasused as the first hard mask 40, and a silicon oxide film 42 was used asthe second hard mask 50, was described. However, the present inventionis not limited thereto, but other films may also be used. However, inthe selection of the material for the hard masks, the films that have alarge etching selectivity between the first hard mask and the film to beprocessed, and between the two hard masks, must be selected consideringthe etching conditions or the like.

In the first embodiment, the case wherein a fluorine main-chain positiveresist for F₂ lithography is used as the positive resists 34 and 46. Inthe present invention, however, the resist is not limited thereto, butother resists can be used. An negative resist may also be used dependingon the pattern of the photomask.

Further in the present invention, the method for forming each film,materials, etching conditions, and exposing conditions are not limitedto those described for the first embodiment. These can be appropriatelyselected as required within the scope of the present invention.

Second Embodiment

FIG. 14 is a top view for illustrating a first photomask in the secondembodiment of the present invention; and FIG. 15 is a top view forillustrating a second photomask in the second embodiment of the presentinvention. FIG. 16 is a schematic diagram for illustrating the statewherein the first photomask is superimposed on the second photomask.

As FIG. 14 illustrates, the first photomask in the second embodiment isan attenuated phase shifting mask. The first photomask is composed of alight-shielding portion 52, real patterns 54 corresponding to linepatterns actually formed on the film to be processed, and dummy patterns56 formed for adjusting the pattern pitch of the first photomask as awhole within a prescribed range.

As FIG. 15 illustrates, the second photomask has a light-shieldingportion 58 and openings 60. The openings 60 are formed in the locationsurrounding the region where the real patterns 54 of the first photomaskare formed. The light-shielding portion 58 is formed in the locationthat covers the dummy patterns 56 and the light-shielding portion 52where no outer patterns are formed in the first photomask.

Therefore, when the second photomask is superimposed on the firstphotomask, the pattern wherein only the area of the real patterns 54 areopened is formed as FIG. 16 illustrates.

The method for forming fine patterns on a film to be processed in thesecond embodiment is the same as the method described for the firstembodiment. Specifically, a first hard mask is formed on a film to beprocessed using a first photomask (Steps S4 to S16); thereafter, asecond hard mask is formed on the first hard mask using the secondphotomask (Steps S18 to S32). Next, the film to be processed is etchedusing the first hard mask and the second hard mask (Step S34), and thefirst and second hard masks are removed (Step S36). Thereby, linepatterns corresponding to the real patterns 54 can be formed on the filmto be processed.

FIG. 17 is a graph for illustrating the relationship between the linesize of the line patterns formed in the second embodiment and defocus;the ordinate showing the pattern size (nm) and the abscissa showingdefocus (μm). In FIG. 17, lines indicated with Line 1 (plotted bylozenge “

”), Line 2 (plotted by square “□”), Line 3 (plotted by triangle “Δ”),and Line 4 (plotted by circle “◯”) show line patterns to which Line 1,Line 2, Line 3, and Line 4 of the first photomask shown in FIG. 14 aretransferred.

As FIG. 17 illustrates, according to the second embodiment, change inthe size of the each of the formed lines is smaller compared with theconventional case shown in FIG. 45, even when a pattern having noperiodicity, for example, the Line 1 on the end of real patterns 54, andthe Line 4 located in the isolated area are defocused. Specifically,according to method to form the fine pattern in the second embodiment,it is seen that the focal depth is significantly improved even in thetransfer of patterns having no periodicity in real pattern.

Since other parts are same as in the first embodiment, the descriptionthereof will be omitted.

Third Embodiment

FIG. 18 is a top view for illustrating a first photomask in the thirdembodiment of the present invention; and FIG. 19 is a top view forillustrating a second photomask in the third embodiment of the presentinvention. FIG. 20 is a schematic diagram for illustrating the statewherein the second photomask is superimposed on the first photomask.

As FIG. 18 illustrates, the first photomask in the third embodiment isan attenuated phase shifting mask having a transmittance of 5%. Thefirst photomask is composed of a light-shielding portion 62, realpatterns 64 corresponding to the patterns actually formed on the film tobe processed, and dummy patterns 66 disposed for adjusting the patternpitch overall the first photomask within a prescribed range. In thefirst photomask, hole patterns of a pattern pitch of 130 nm includingthe real patterns 64 and the dummy patterns 66 are formed.

As FIG. 19 illustrates, the second photomask in the third embodiment hasa light-shielding portion 68 and openings 70. The openings 70 are formedso as to open on the regions where the real patterns 64 of the firstphotomask are formed. The light-shielding portion 68 is formed so as tocover the dummy patterns 66 and the surrounding light-shielding portion62 of the first photomask.

Therefore, when the second photomask is superimposed on the firstphotomask, the pattern that opens only on the area of the real pattern64, as FIG. 20 illustrates, is formed.

When a photomask having such hole patterns is used, the method forforming fine patterns on the film to be processed is the same as themethod described in the first embodiment. The third embodiment will bedescribed in detail below.

First, a low-dielectric-constant film as a film to be processed isformed on a substrate (Step S2). Thereafter, a first hard mask is formedon the low-dielectric-constant film using the first photomask in thethird embodiment (Steps S4 to S16), and then, a second hard mask isformed on the low-dielectric-constant film and the first hard mask usingthe second photomask in the third embodiment (Steps S18 to S32).Thereafter, the low-dielectric-constant film is etched using the firsthard mask and the second hard mask as masks (Step S34), and the firstand second hard masks are removed. Thus, the hole patterns thatcorrespond to the real patterns 64 of the first hard mask can be formedon the low-dielectric-constant film.

However, in the third embodiment, the thickness of thelow-dielectric-constant film is 250 nm. In addition, the patterns to beformed are hole patterns. Therefore, when exposure using the firstphotomask having regularly arrayed hole patterns (Step S10) isperformed, a quadrapole illumination light source of a center sigma (σ)of 0.4, and the radius of σ is 0.05 is used as an illumination lightsource.

As described above, also when the patterns to be processed are holepatterns, first, periodical patterns are accurately formed using a phaseshifting mask and off-axis illumination, and then, a second hard maskthat covers unnecessary areas is formed. Then, etching is performedusing the first hard mask and the second hard mask as a double-layermask. Thereby, accurate patterns can be formed even when the patterns tobe formed are fine hole patterns.

In the third embodiment, the case wherein a quadrapole illuminationlight source is used as the illumination light source for forming holepatterns was described. However, as described for the first embodiment,the conditions of illumination are not limited thereto, but can besuitably selected considering the type, the pattern shape and the likeof the first photomask.

Since other parts are the same as in the first embodiment, thedescription thereof will be omitted.

Fourth Embodiment

FIG. 21 is a flow diagram for illustrating the method for formingpatterns according to the fourth embodiment of the present invention.FIGS. 22 to 27 are schematic sectional views for illustrating the statesin the process for forming patterns according to the fourth embodimentof the present invention.

In the fourth embodiment, the photomasks used for forming patterns arethe first and second photomasks similar to those described for the firstembodiment. The fine patterns to be formed are also similar to thetrench pattern 24 described for the first embodiment.

In the first embodiment, however, the first hard mask 40 and the secondhard mask 50 are used in the etching of the low-dielectric-constantinsulating film 22; while in the fourth embodiment, two layers of resistmasks are used for etching. Specifically, a first resist mask is formedusing the first photomask, a second resist mask is formed using thesecond photomask, and the low-dielectric-constant insulating film 22 isetched using these photomasks as masks. The fourth embodiment will bedescribed below in detail.

First, as FIG. 22 illustrates, in the same manner as Step S2 of thefirst embodiment, a low-dielectric-constant insulating film 22, which isa film to be processed, is formed on a substrate 20 (Step S40).Thereafter, an organic anti-reflection coating 72 is formed on thelow-dielectric-constant insulating film 22 (Step S42), and a positiveresist 74 is spin-coated (Step S44). The organic anti-reflection coating72 is selected from materials having a sufficiently large etchingselectivity to the positive resist 74 considering subsequent etchingconditions.

Thereafter, exposure, developing treatment, and baking are performedusing the first photomask described for the first embodiment as the mask(Steps S48 to S52). Thereby, as FIG. 23 shows, the positive resist 74 ispatterned, and the first resist mask 80 having openings (real) 76 andopening (dummy) 78 that correspond to the real patterns 4 and the dummypatterns 6 of the first photomask, respectively, are formed. Here, theexposure conditions and the like are the same as those described for thefirst embodiment.

Next, as FIG. 24 illustrates, an organic anti-reflection coating 82 isformed on the first resist mask 80 and the organic anti-reflectioncoating 72 so as to bury openings 76 and 78 (Step S54). The first resistmask 80 is buried with the organic anti-reflection coating 82, so thatthere are no irregularities due to the first resist mask 80 on thesurface of the organic anti-reflection coating 82. Here, the organicanti-reflection coating 82 is selected from materials having asufficiently large etching selectivity to the first resist mask 80considering subsequent etching conditions. The thickness of the organicanti-reflection coating 82 is a thickness that can sufficiently absorblight used for subsequent exposing step. Thereafter, a photoresist 84 isapplied onto the organic anti-reflection coating 82 using spin coating(Step S56).

Next, exposure, developing treatment, and baking are performed using thesecond photomask described for the first embodiment as the mask (StepsS58 to S62). Here, the exposure conditions and the like are also thesame as those described for the first embodiment. Thereby, a secondresist mask 88 having an opening 86 that opens on the area of theopenings (real) 76 of the first resist mask 80 is formed. Here, theorganic anti-reflection coating 82 of a thickness that can sufficientlyabsorb exposure light is formed on the first resist mask 80. Therefore,in this exposure, the exposure of the first resist mask 80 together withthe positive resist 84 can be prevented.

Next, as FIG. 26 illustrates, the organic anti-reflection coatings 72and 82 that expose on the bottom of the opening 86 is etched using thesecond resist mask 88 as a mask (Step S64). Thereafter, thelow-dielectric-constant insulating film 22 is etched using the firstresist mask 80 and the second resist mask 88 as a mask (Step S66). Here,in the state of FIG. 26 wherein the first resist mask 80 and the secondresist mask 88 overlap with each other, the surface of thelow-dielectric-constant insulating film 22 is exposed only to openings(real) 76 that correspond to real patterns. Therefore, as FIG. 27illustrates, the area of the openings (real) 76 is etched, and trenchpatterns 24 are formed in the low-dielectric-constant insulating film22.

Thereafter, the first resist mask 80 and the second resist mask 88 areremoved (Step S68). Thereby, fine patterns similar to those of the firstembodiment shown in FIG. 3 can be formed.

In the fourth embodiment, as described above, resist masks are used inplace of the hard masks. Therefore, in the first embodiment, afterresist patterns have been once formed, the hard masks are etched offusing the resist patterns as masks; while in the fourth embodiment,first and second resist masks are formed, and these resist masks aredirectly used as the masks for etching the low-dielectric-constantinsulating film 22. Therefore, the number of steps for forming finepatterns can be reduced, and the throughput of semiconductor devices orliquid-crystal devices can be improved.

Also in the fourth embodiment, an attenuated phase shifting mask havinga constant pattern pitch is used in the formation of the first resistmask, and a dipole illumination light source is used as the exposinglight source. Therefore, the positive resist 72 can be accuratelypatterned. The second resist mask 88 is formed so as to cover theunnecessary area of the first resist mask 80. Therefore, withcombination of the first and second photomasks, masks having finepatterns can be accurately formed, and accurate pattern formation can beperformed.

In the fourth embodiment, an organic anti-reflection coating 82 isformed before applying the positive resist 84 for forming the secondresist mask 88 (Step S54). Thus, by the formation of the organicanti-reflection coating 82, the irregularity formed by the first resistmask 80 can be planarized, and the positive resist 84 can be evenlyapplied. Furthermore, by the formation of the organic anti-reflectioncoating 82 having a sufficient thickness, the exposure light can beabsorbed to prevent the simultaneous exposure of the first resist mask80 in the exposure of the positive resist 84. Therefore, by theformation of the organic anti-reflection coating 82, the more accurateformation of fine patterns can be realized.

However, the present invention is not limited to the case wherein theorganic anti-reflection coating 82 having a thickness to absorb exposurelight is formed. In the present invention, for example, two types ofresists having different photosensitivity can be used for the firstresist mask 80 and the second resist mask 88, respectively. Thereby, theexposure of the first resist mask 80 can be prevented in the exposurewhen the second resist mask 88 is formed.

In the fourth embodiment, although resist masks are used together withthe first and second masks, the present invention is not limitedthereto. For example, the first hard mask 40 can be formed in the samemanner as in the first embodiment (Steps S4 to S16) for only thefirst-layer mask that requires exposure and development of finerpatterns; and the resist mask 88 as described for the fourth embodimentcan be used for the second-layer mask. By so doing, in the formation ofthe second-layer mask having relatively small number of fine patterns,the step of etching the material film for the hard mask can beeliminated, and more accurate fine patterns can be formed whileimproving throughputs.

For example, generally when hard masks are used as in the firstembodiment, the hard masks themselves can be relatively thinned if theetching selectivity of the hard mask to the film to be processed isselected to be large. Therefore, the film thickness of the resist usedfor forming the hard mask can also be thinned, and accurate exposure canbe performed. Whereas, in order to directly process a relatively thickfilm to be processed using resist masks as in the fourth embodiment, thethickness of the resist must be secured to some extent. Therefore, theaccuracy of fine processing is inferior to the accuracy in the first tothird embodiments wherein hard masks are used. On the other hand, whenresist masks are used, the number of steps can be reduced than the casesusing hard masks.

Therefore, when fine patterns are formed, whether the use of hard masks,the use of resist masks, or the use of a hard mask as the first mask anda resist mask as the second mask can be selected considering accuracynecessary for pattern processing, productivity or the like.

Since other parts are the same as in the first embodiment, thedescription thereof will be omitted.

Fifth Embodiment

FIG. 28 is a schematic sectional view for illustrating the method formanufacturing a semiconductor device according to the fifth embodimentof the present invention. FIG. 29 is a flow diagram for illustrating themethod for manufacturing the semiconductor device according to the fifthembodiment of the present invention. FIGS. 30 to 43 are schematicsectional views for illustrating the state in each step formanufacturing the semiconductor device according to the fifth embodimentof the present invention.

In the fifth embodiment, the semiconductor device having ainterconnection structure formed by a single damascene process ismanufactured using the methods for forming fine patterns in the first tofourth embodiments described above. The fifth embodiment will bespecifically described referring to FIGS. 28 to 44.

As FIG. 28 shows, a transistor 92 is formed on the substrate 90 of thesemiconductor device. A silicon oxide film 94 that covers the transistor92 via a silicon nitride film is also formed on the substrate 90. Thesilicon oxide film 94 is an interlayer insulating film, and thethickness thereof is about 600 nm. In the silicon oxide film 94 isformed a contact plug 96 extending to the source-drain region of thetransistor 92. The contact plug 96 is composed of tungsten 102 buried inthe contact hole 98 through a barrier metal 100 consisting of a titaniumnitride layer and a titanium layer.

A low-dielectric-constant insulating film 104 is formed on the siliconoxide film 94. The thickness of the low-dielectric-constant insulatingfilm 104 is about 130 nm. Passing through the low-dielectric-constantinsulating film 104, a metal wiring 106 that is connected to the contactplug 96 is formed. The metal wiring 106 is composed of copper (Cu) 112buried in the hole 108 through a barrier metal 110 consisting of atantalum nitride layer and a tantalum layer.

A low-dielectric-constant insulating film 114 is formed on thelow-dielectric-constant insulating film 104. The thickness of thelow-dielectric-constant insulating film 114 is about 250 nm. Passingthrough the low-dielectric-constant insulating film 114, a via plug 116that is connected to the metal wiring 106 is formed. The via plug 116 iscomposed of copper 122 buried in the via hole 118 through a barriermetal 120 consisting of a tantalum nitride layer and a tantalum layer.

When the semiconductor device constituted as described above ismanufactured, required openings in the silicon oxide film 94, and thelow-dielectric-constant insulating films 104 and 114 are formed usingthe methods described for the first to fourth embodiments. This will bespecifically described below.

First, a gate, a source-drain region, and the like are formed on thesubstrate 90 to form the transistor 92 (Step S102).

Next, the silicon oxide film 94, which is an interlayer insulating filmthat covers the transistor 92 via a silicon nitride film thereon, isformed on the substrate 90 (Step S104). Here, a silicon oxide film 94 isdeposited using a plasma CVD method, and is planarized using CMP.Planarization is performed so that the thickness of the silicon oxidefilm 94 from the surface of the substrate 90 becomes about 600 nm afterplanarization.

Next, the contact hole 98 is formed in the silicon oxide film 94 (StepS106). The contact hole 98 is formed using the method described for thefourth embodiment.

In the same manner as in Steps S4 to S16 of the first embodiment, afirst hard mask is formed. Specifically, as FIG. 30 illustrates, asilicon nitride film 130 of a thickness of 80 nm, an organicanti-reflection coating 132, and a positive resist 134 are formed on thesilicon oxide film 94 of the first embodiment. Thereafter, the positiveresist 134 is exposed and developed using a first photomask. Here, asthe first photomask, a photomask having real patterns that correspond tothe contact hole 98, and dummy patterns arranged for adjusting thepattern pitch and density is used. Furthermore, as illustrated in FIG.31, the organic anti-reflection coating 132 and the silicon nitride film130 are etched using the positive resist film 134 as a mask. Then, thepositive resist 134 and the organic anti-reflection coating 132 areremoved. Thereby, there is formed a first hard mask 140 having anopening (real) 136 on the location where the contact hole 98 is formed,and openings (dummy) 138 corresponding to dummy patterns of the firstmask.

Next, in the same manner as in Steps S54 to S64 of the fourthembodiment, a second resist mask is formed. Specifically, as FIG. 32illustrates, an organic anti-reflection coating 144 is formed on thesilicon oxide film 94 and the first hard mask 140. The organicanti-reflection coating 144 is formed so as to cover the irregularity ofthe first hard mask 140 to planarize the surface to some extent.Thereafter, a positive resist 146 is formed on the organicanti-reflection coating 144. Next, the positive resist 146 is exposedand developed using the second photomask. As the second photomask, aphotomask having an opening in the location surrounding the region wherethe real patterns of the first photomask are formed, and alight-shielding portion in the location that corresponds to the regionwhere the dummy patterns are formed is used. Thereafter, as FIG. 33illustrates, the organic anti-reflection coating 144 is etched using thepositive resist 146 as a mask. Thereby the second resist mask 150 havingan opening 148 on the opening (real) 136 is formed.

Next, the silicon oxide film 94 is etched using the first hard mask 140and the second resist mask 150 as masks, and after etching, the firsthard mask 140 and the second resist mask 150 are removed. Thereby, asFIG. 34 illustrates, a contact hole 98 is formed in the silicon oxidefilm 94.

Next a two-layer film of titanium and titanium nitride is deposited as abarrier metal 100 in the contact hole 98 (Step S108). Further, tungsten102 is buried in the contact hole 98 (Step Silo), and etched back so asto expose the silicon oxide film 94 on the surface (Step S112). Thereby,as FIG. 35 illustrates, a contact plug 96 connected to the source-drainregion is formed in the silicon oxide film 94.

Next, a low-dielectric-constant insulating film 104 is formed on thesilicon oxide film 94 (Step S114). The low-dielectric-constantinsulating film 104 is deposited using a plasma CVD method to have athickness of 130 nm. Thereafter, fine holes are formed in thelow-dielectric-constant insulating film 104 (Step S116). For theformation of the fine holes, the method for forming patterns describedfor the first embodiment is used.

Specifically, first, a first hard mask is formed on thelow-dielectric-constant insulating film 104 in the same manner as inSteps S4 to S16 of the first embodiment. Here, after forming a siliconnitride film 230 of a thickness of 80 nm on the low-dielectric-constantinsulating film 104 as the material film for the first hard mask, theorganic anti-reflection coating 232 and the positive resist 234 areformed on the silicon nitride film 230. Thereafter, exposure isperformed using the first photomask as a mask. Here, the photomaskhaving real patterns that correspond to the holes 108, and dummypatterns arranged for uniforming the pattern pitch and density on theentire photomask is used as the first photomask. Here, similar to thethird embodiment, a quadrapole illumination light source of a centersigma (σ) of 0.4, and the diameter of σ of 0.05, using an F₂ excimerlaser of a wavelength of 157 nm as a exposing light source, is used. Thenumerical aperture NA of the lens is 0.95.

Thereafter, the developing treatment of the positive resist 234 isperformed to form a resist pattern, and the organic anti-reflectioncoating 232 and the silicon nitride film 230 are etched using the resistpattern as a mask. Thereby, as FIG. 36 illustrates, an opening (real)236 that corresponds to the hole 108, and openings (dummy) 238 foruniforming the pattern pitch are formed. Thereafter, the positive resist234 and the organic anti-reflection coating 232 are removed (Step S16).Thereby, a first hard mask 240 is formed on the low-dielectric-constantinsulating film 104.

Next, as FIG. 37 illustrates, a second hard mask is formed on thelow-dielectric-constant insulating film 104 in the same manner as inSteps S18 to S32 of the first embodiment. Here, a silicon oxide film 242of a thickness of 30 nm is formed on the surfaces of thelow-dielectric-constant insulating film 104 and the first hard mask 240as the material film for the second hard mask, and an organicanti-reflection coating 244 and the positive resist 246 are formedthereon.

Next, exposure treatment is performed. The second photomask used here isa chromium mask, which isolates the real patterns from dummy patterns inthe first hard mask to open only the area of real patterns. Afterexposure, developing treatment is performed to form resist patterns, andthe organic anti-reflection coating 244 and the silicon oxide film 242are etched using the resist patterns as masks. Thereby, as FIG. 38illustrates, an opening 248 is formed on the opening (real) 236.Thereafter, the positive resist film 246 and the organic anti-reflectioncoating 244 are removed. Thereafter, the positive resist 244 and theorganic anti-reflection coating 246 are removed Thereby, the second hardmask 250 is formed.

Next, as FIG. 39 illustrates, the low-dielectric-constant insulatingfilm 104 is etched using the first hard mask 240 and the second hardmask 250 as masks. Thereafter, the first and second hard masks 240 and250 are removed. Thereby, a hole 108 is formed in thelow-dielectric-constant insulating film 104.

The conditions of exposure, etching or the like for forming the hole 108are the same as those in the first embodiment unless otherwisespecified.

Next, a barrier metal 110 is formed on the inner wall of the hole 108(Step S18). The barrier metal 110 is formed by vapor-depositing thetwo-layer film of tantalum nitride and tantalum using plasma CVD.Thereafter, copper 112 is buried in the hole 108 using an electrolyticplating method (Step S120), and is planarized using CMP (Step S122).Thereby, as FIG. 40 illustrates, a metal wiring 106 to be connected tothe contact plug 96 is formed in the low-dielectric-constant insulatingfilm 104.

Next, a low-dielectric-constant insulating film 114 is formed on thelow-dielectric-constant insulating film 104 (Step S124). Thelow-dielectric-constant insulating film 114 is accumulated using aplasma CVD method to have a thickness of about 250 nm. Thereafter, a viahole 118 is formed in the low-dielectric-constant insulating film 114(Step S126). In the formation of the via hole 118, the same methods asin Steps S4 to S36 in the first embodiment are used similar to theformation of the hole 108.

Specifically, a silicon nitride film 330, an organic anti-reflectioncoating 332, and a positive resist 334 are formed on thelow-dielectric-constant insulating film 114. Thereafter, exposure anddeveloping treatment is performed using the first photomask having realpatterns that correspond to the via hole 118 and dummy patterns forcontrolling the pattern pitch, and, as illustrated in FIG. 41, furtheretching or the like of the organic anti-reflection coating 332 and thesilicon nitride film 330 are performed using the positive resist 334 asa mask. Thus, the first hard mask 340 having an opening (real) 336 thatcorresponds to real patterns and openings (dummy) 338 that correspondsto dummy patterns is formed.

After removing the positive resist 334 and the organic anti-reflectioncoating 332, a silicon oxide film 342, an organic anti-reflectioncoating 344, and a positive resist 346 are formed on thelow-dielectric-constant film 114 and the first hard mask 340, and thepositive resist 346 is exposed and developed using the second photomask.Furthermore, as FIG. 42 illustrates, the organic anti-reflection coating344 and the silicon oxide film 342 are etched using the positive resist346 as the mask to form an opening 348. Thus the second hard mask 350having an opening 348 that opens on the region where the opening (real)336 is formed and that covers the region where the openings (dummy) 338are formed is formed.

After removing the positive resist 346 and the organic anti-reflectioncoating 344, as FIG. 43 illustrates, the low-dielectric-constantinsulating film 114 is etched using the first and second hard masks 340and 350 as masks, and then, the first and second hard masks 340 and 350are removed after forming the via hole 118.

The conditions of exposure, etching or the like for forming the via hole118 are the same as those in the first embodiment unless otherwisespecified.

Next, a barrier metal 120 consisting of a two layer film of tantalumnitride and tantalum is formed on the inner wall of the via hole 118(Step S128), copper is buried using electrolytic plating (Step S130),and planarized using CMP (Step S132). Thereby, as FIG. 28 illustrates, avia plug 116 is formed in the low-dielectric-constant insulating film114.

As described above, a semiconductor device having a multi-layer wiringlayer of a single Damascene structure is formed. Another wiring layermay further be laminated on the upper layer as required.

According to the fifth embodiment, as described above, two layers ofmasks described in the first to fourth embodiments are formed, andpatterns are formed using these two layers of masks. Therefore, finepatterns can be formed faithfully to the pattern design, and a highlyreliable semiconductor device can be obtained.

In the fifth embodiment, a contact plug 96, a metal wiring 106, and avia plug 116 are shown in the drawings for simplification. However, thepresent invention is not limited thereto, but required wiring layers canbe formed on the required locations in the same manner as the fifthembodiment.

Also in the fifth embodiment, the formation of a semiconductor devicehaving a multi-layer wiring layer of a single Damascene structure wasdescribed. However, the present invention can be widely applied to thecases wherein fine patterns must be formed, such as in the manufactureof other semiconductor devices and liquid-crystal devices.

Also in the fifth embodiment, the case wherein the combination of thehard mask and the resist mask was used for etching the silicon oxidefilm 94 in the formation of the contact hole 98 was described.Furthermore, in the formation of the hole 108 and the via hole 118, theuse of two layers of hard masks was described. However, the presentinvention is not limited thereto, but the combination with other masks,such as the combination of two layers of hard masks described in thefirst embodiment, the combination of two layers of resist masksdescribed in the fourth embodiment, and the combination of a hard maskand a resist mask, may also be used. These masks may be optionallyselected considering the size of required patterns, productivity, andthe like, as described above. However, for example, when etching isperformed using a combination of hard masks, the material having asufficiently large etching selectivity with the film to be processed(e.g., the silicon oxide film 94 and the like).

For example, by carrying out Step S2 of the first embodiment or Step S40of the third embodiment, the step for forming the film to be processedof the present invention is carried out. Also for example, by carryingout Steps S4 to S16 of the first embodiment, or Steps S42 to S52 of thethird embodiment, the step for forming the first mask of the presentinvention is carried out; and by carrying out Steps S18 to S32 of thefirst embodiment, or Steps S54 to S62 of the third embodiment, the stepfor forming the second mask of the present invention is carried out. Bycarrying out Steps S34 or S66 of the first or third embodiments, theetching step of the present invention is carried out.

Also for example, in the first embodiment, by carrying out Steps S4 andS18, the steps for forming the first and second material films of thepresent invention are carried out, respectively; by carrying out StepsS8 to S12 and S22 to S26, the steps for forming the first and secondresist patterns of the present invention are carried out, respectively;and by carrying out Steps S14 and S30, the first and second etchingsteps are carried out, respectively.

Also for example, in the third embodiment, by carrying out Steps S44 andS56, the first and second resist applying steps are carried out,respectively; by carrying out Steps S48 to 52 and S56 to 62, the stepsfor forming the first and second resist masks are carried out,respectively; and by carrying out Step S64, the step for etching theanti-reflection coating is carried out.

The features and the advantages of the present invention as describedabove may be summarized as follows.

According to one aspect of the present invention, a first and a secondphotomask are used in exposure. The mask formed by combining the firstand second masks formed on a film to be processed using the first andsecond photomasks is a mask opened only on the area of an actuallyformed real pattern. The film to be processed is etched using the firstand second masks to form a desired pattern. Here, in the first mask,since the pattern pitch is controlled within a prescribed range, thepattern can be accurately formed even when a technique requiringperiodicity in the pattern to some extent, such as a resolutionenhancement technique is used. In addition, the second mask masks theunnecessary area of the pattern. Therefore, the pattern having the areaout of periodicity can be formed faithfully to the pattern design.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay by practiced otherwise than as specifically described.

The entire disclosure of a Japanese Patent Application No. 2003-377439,filed on Nov. 6, 2003 including specification, claims, drawings andsummary, on which the Convention priority of the present application isbased, are incorporated herein by reference in its entirety.

1. A method for forming patterns comprising: forming, on a substrate, afilm to be processed; forming a first mask, which is a hard mask, onsaid film to be processed, by lithography, using a first photomask,wherein forming said first mask includes forming a first material filmthat becomes the material film of said first mask on said film to beprocessed, forming a first resist pattern by transferring the pattern ofsaid first photomask onto said first material film, using lithography,said first photomask having a real pattern, which is an actual patternon said film to be processed, and a dummy pattern added to controlpattern pitch in said first photomask to within a prescribed range forresolution enhancement of the real pattern of the first photomask, andetching said first material film using said first resist pattern as anetching mask; forming a second mask, which is a hard mask, on said filmto be processed and on said first mask, by lithography, using a secondphotomask, wherein forming said second mask includes forming a secondmaterial film that becomes the material film of said second mask on saidfilm to be processed and on said first mask, forming a second resistpattern by transferring the pattern of said second photomask onto saidsecond material film, using lithography, said second photomask having apattern isolating a region in which said dummy pattern is formed from aregion in which said real pattern is formed, and etching said secondmaterial film using said second resist pattern as an etching mask; andetching said film to be processed using said first mask and said secondmask as etching masks.
 2. The method for forming patterns according toclaim 1, wherein said first mask is a silicon nitride film and saidsecond mask is a silicon oxide film.
 3. The method for forming patternsaccording to claim 1, including using a quadrapole illumination sourcefor exposure in the lithography used in forming said first mask.
 4. Themethod for forming patterns according to claim 1, including using adipole illumination source for exposure in the lithography used informing said first mask.
 5. The method for forming patterns according toclaim 1, including using a circular illumination source for exposure inthe lithography used in forming said first mask.
 6. The method forforming patterns according to claim 1, wherein said first photomask isan attenuated phase shifting mask.
 7. The method for forming patternsaccording to claim 1, wherein said first photomask is an alternatingphase shifting mask.